President Trump’s threat to “send in the feds” to Chicago if the city is unable to reduce violent crime is stirring consternation among law enforcement and policing experts, who say the president’s comments could be interpreted broadly as anything from sending in the National Guard to increasing federal funding for law enforcement.

Responding to the president’s criticism on Wednesday, Chicago Mayor Rahm Emanuel said that while he welcomes federal involvement and funding, the notion of sending the National Guard into the city is “antithetical” to the spirit of community policing and trust he’s working to repair.

“Chicago, like other cities that are dealing with gun violence, wants the partnership with federal law enforcement entities in a more significant way than we [have] today — whether that’s the FBI, the DEA and the ATF,” Mr. Emanuel said, suggesting that investment in schools and mentoring children also need to be part of the equation.

White House press secretary Sean Spicer sought to clarify the president’s comments Wednesday, saying his concern was spurred by “turning on the television and seeing Americans get killed by shootings.”

“What he wants to do is provide the resources of the federal government, and it can span a bunch of things. There’s no one thing,” Mr. Spicer said, citing federal law enforcement aid that could be requested by Illinois’ governor or provided to the state’s U.S. Attorney’s Offices.

Mr. Spicer said any next steps in addressing the city’s violence will involve a dialogue with the mayor “to figure out what a path forward can be.”

While the federal government can take varied actions to help local police combat crime, policing experts say one thing is clear — the federal government can’t simply take over as the chief law enforcement agency in the city.

“The feds don’t do policing,” said Daniel Nagin, a criminologist and professor of Public Policy and Statistics at Carnegie Mellon University’s Heinz College. “A critical ingredient to getting this under control is going to require aggressive police action. Only the Chicago police can do that.”

James Pasco, executive director of the National Fraternal Order of Police, said Wednesday that he interprets Mr. Trump’s comments as directing more federal law enforcement to assist Chicago police, not instructing them to take over.

“The feds can be tremendously helpful, and we support the president’s call for assistance,” said Mr. Pasco, noting that Chicago police have the principal responsibility and are best equipped to lead the efforts to reduce crime. “They [feds] are there to assist, not to take over.”

The Chicago Police Department is in the midst of what could be a major overhaul, and in recent months has suffered from a lack of community trust as a result of high-profile law enforcement shootings of black civilians.

‘Not immediately clear’

A Department of Justice report released just before Mr. Trump’s inauguration concluded officers were quick to use excessive and deadly force, failed to de-escalate tense situations and engaged in other behavior that not only violated constitutional rights, but diminished the department’s ability to fight crime. After the release of critical reports in other cities, a federal judge has often overseen implementation of police reform plans, but it is unclear what action the Trump administration will take.

Chicago Police Superintendent Eddie Johnson on Wednesday told the Chicago Tribune that he was baffled by the unspecified nature of Mr. Trump’s comments about sending in the federal government.

“The statement is so broad. I have no idea what he’s talking about,” said Superintendent Johnson.

The city’s top cop said he would oppose any plan that included deploying the National Guard to quell the violence, but he would be supportive of federal partnerships aimed at prosecuting more gun crimes.

Tim Lynch, director of the Cato Institute’s Project on Criminal Justice, said the federal law enforcement agencies could all be asked to step up coordination with the Chicago Police Department, and likely have already done so.

Mr. Lynch suggested the U.S. Marshals Service, for instance, could prioritize cases in order to help locate potential witnesses to unsolved homicides in an effort to make arrests while the Federal Bureau of Investigation or the Bureau of Alcohol, Tobacco, Firearms and Explosives might add extra resources in the Chicago area to target illegal gunrunners or wanted offenders police believe are tied to ongoing violence in order to get them off the street.

“This has already been going on to some extent though, so when Mr. Trump says ‘I will take further action,’ it’s not immediately clear what he means,” he said. “If it goes beyond, then it could be very disturbing in terms of what he perceives the role of the federal government to be in situations like this.”

While Mr. Spicer suggested federal aid for law enforcement could be made available, the city is at risk of losing federal dollars allocated for other purposes as a result of its status as a so-called “sanctuary city” — one of hundreds of jurisdictions across the country that limit cooperation with federal immigration enforcement agents.

Mr. Trump on Wednesday signed an executive order that would deny federal grants, except those deemed necessary for law enforcement purposes, to any sanctuary jurisdiction.

Mr. Emanuel, who previously served as former President Obama’s chief of staff, said Wednesday that Chicago would remain a sanctuary city.

Freddy Martinez, director of Lucy Parsons Labs, which has advocated more oversight and transparency in the Chicago Police Department, said Mr. Trump’s and Mr. Emanuel’s stances on the issue are worrisome.

“Whatever happens with sanctuary cities and possible defunding of programs, that is going to hit the things that [are] driving the inequality and the things driving the violence,” Mr. Martinez said.

Of the funding destined for police the Trump administration might allocate, Mr. Martinez said he doesn’t think it will quell the violence.